Method and apparatus for stirring and treating continuous and semi continuous metal casting

ABSTRACT

A method and apparatus for stirring and treating continuous and semi-continuous casting of metallic rods ( 12 ). The apparatus comprising: a stand ( 30 ) for suspending at least one electric arc electrode ( 24 ) over the upper surface of a metallic casting ( 14 ) after or during pouring, at least one electrode ( 24 ) for forming a moving electric arc ( 38 ) over the upper surface of the mettalic casting being cast, a second electrode, being the liquid metal ( 14 ), for completion of an electric circuit including said electric arc ( 38 ) and the liquid metal ( 14 ); and control means ( 34 ), connected between said apparatus and a power supply ( 36 ) and arranged to supply power to the plasma arc electrodes ( 14, 24 ), for monitoring the parameters of the electric arc ( 38 ). The invention also provides a method for casting the metallic rods ( 12 ) by using the above described apparatus.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to metal continuous and semi continuouscastings. More particularly, the invention provides an improved methodand apparatus for stirring and treating continuous and semi-continuouscasting to produce a high-quality cast of any desired length and crosssection composed of a ferrous or of a non-ferrous metal.

Continuous and Semi-continuous casting are the most commonly usedmethods for casting and producing semi-finished products such as plats,tubs, shits, etc. While continuous casting is the main method forcasting ferrous alloys i.e. low carbon steel, stainless steel, etc.,semi-continuous casting is the preferred method for casting non-ferrosalloys such as aluminium and copper alloys. Both methods based oncontinuously pouring metal from a reservoir mostly known as the tundishor the distributor, to a cooled open mold. The mold is open both formetal entrance (usually from the top) and for metal exit (usually fromthe bottom). Unlike other casting methods the metal does not solidifiescompletely in the mold, but rather receives an initial solidifying shelland continues to cools outside the mold most commonly by water jets. Thecast produced by continuous or semi-continuous casting such as billet,bloom, bar, slab etc, hereandafter defined as “rod” is advanced by meansof rollers, gravitation, piston etc. the rod undergo further plasticdeformation for producing the semi-finished products, mentioned above.The main difference between continuous and semi-continuous castingregards the length been cast. In continuous casting the total length canbe hundreds of meters in a continuous process. At the end of the castingthe rods, are cut from the casting. While in semi-continuous casting thelength is constant in the size of several meters, and each billet iscast in a single process.

A further mentions of continuous casting will refer to continuous aswell as semi continuous casting.

A further effect in metal alloy casting is the appearance of dendritesduring cooling, these being formed during solidification as variouspoints in the mass take up a lattice tree like structure. During theformation of dendrites, alloying elements, such as C, Cr or Ni arepushed outwardly to form a crystal grain boundary, these form later asite for the initiation of cracks in a finished component. Aconcentration of these alloying elements is referred to as segregation,which can to some extent be combated by lower pouring temperatures.

Impurities from the ladle or the casting powder can form inclusions andfurther reduce the mechanical properties of the products.

Gases, from the atmosphere or other sources are also present in theliquid metal, these being the main cause of casting porosity.

Applying roller pressure to the rod during continuous casting isproposed by Fukuoka et al. in Japanese Patent no. JP56050705A2. Pressureis said to prevent the generation of a crack on the bottom side of thecasting groove. The roller is located at the point where the bent ingotis straightened. Obviously this process is of no help in improving themicrostructure of the metal.

In U.S. Pat. No. 4,756,749 Praitoni et al describe and claim a processfor the continuous casting of steel from a tundish having severalcasting spouts. While in the tundish the molten steel is subjected tofurther heating, which in claim 5 is a transferred-arc plasma torch.This patent is for heating the metal in the tundish, while the presentinvention is for treating the molten metal in the mold during thesolidification process.

Lowry et al in U.S. Pat. No. 4,770,724 describe an unusual continuouscasting method for metals which claims to eliminate voids and flaws andto produce a dense homogeneous product. This is achieved by forcing themetal to flow upwards, against gravity, by means of an electromagneticfield which also provides containment forces. The method disclosedincludes electromagnetic stirring of the molten metal; this aspect ofthe process is in common industrial use, and is known to improvehomogeneity and produce a fine grain structure.

However electromagnetic stirring consumes much electric power, cannot beapplied to very large castings and has limited influence on ingotshaving aspect ratios (width/breadth ratio) in excess of 1.8

In U.S. Pat. No. 5,963,579 Henryon discloses a method of heating amolten metal in a continuous casting tundish, using a plasma torch and aspecial tundish. Other prior-art casting systems are referred to in thisspecification which also apply a plasma arc to metal held in thetundish. As with the Praitoni disclosure, such heating and/or stirringdoes not effect the solidification process in the mold.

PCT/IL97/00023 describes an electric arc which could be used in thepresent aparatus.

OBJECTS OF THE INVENTION

It is therefore one of the objects of the present invention to obviatethe disadvantages of prior art for stirring and treating continuouscasting rods and to provide an improved method and an apparatus forproducing better quality rods, without the need to consume greatquantities of electric power.

It is a further object of the present invention to provide an apparatusthat will stir the metal during the solidification process and thus,increase homogeneity, decrease gas porosity and break up dendrites intosmall pieces and thereby reduce the grain size of the finished rod.

Yet a further object of the invention is to improve the production of acontinuous rods having aspect ratios exceeding 1.8.

SUMMARY OF THE INVENTION

The present invention achieves the above objects by providing a stirringand treating continuous casting apparatus for the production ofimproved-quality rods from molten metal, including rod cross-sectionshaving a aspect ratio exceeding 1.8, the apparatus comprising:

-   -   a) a stand for suspending at least one electric arc electrode        over the upper surface of a metallic casting after or during        pouring;    -   b) at least one electrode for forming a moving electric arc over        the upper surface of said metallic casting being cast:    -   c) a second electrode, being the liquid metal, for completion of        an electric circuit including said electric arc and the liquid        metal; and    -   d) control(s) mean(s) connected between said apparatus and a        power supply and arranged to supply power to the plasma arc        electrode(s) for monitoring the parameters of the electric arc.

The invention also provides a method for improving quality, such as:reducing voids, inclusions, porosity, dendrite and grain size inmetallic continuous rod casting and for improving homogeneity therein,the method comprising

-   -   step a) continuously pouring the liquid metal into a mold for        casting rods.    -   step b) providing at least one first electric arc electrode and        positioning same slightly above the upper surface of the molten        metal;    -   step c) providing a second electrode in electrical contact with        a section of the mold being electrically conductive, and        applying an electric current to said electrodes to form an arc        between said first electrode and said upper surface; and    -   step d) continuously moving said electric arc(s) over said upper        surface.

Yet further embodiments of the invention will be described hereinafter.

It will thus be realized that the novel device of the present inventionimproves rod quality by stirring the liquid metal in the mold. Theelectric power required for this purpose is moderate. For example, in atest carried out in producing a semi-continuous slab, aspect ratio 2,made of copper nickel alloy, the plasma-arc consumed only 2 kWh per ton.

It is to be stressed that the method and apparatus of the presentinvention have been tested in practice, and castings produced by the newmethod have been examined and compared to the same castings produced byconventional methods. Test results, also to be found in the description,leave no doubt as to the improved quality achieved by use of apparatusconstructed according to the present invention.

SHORT DESCRIPTION OF THE DRAWINGS

The invention will now be described further with reference to theaccompanying drawings, which represent by example preferred embodimentsof the invention.

Structural details are shown only as far as necessary for a fundamentalunderstanding thereof. The described examples, together with thedrawings, will make apparent to those skilled in the art how furtherforms of the invention may be realized.

In the drawings:

FIG. 1 is a partially section of a preferred embodiment of the apparatusaccording to the invention;

FIG. 2 is a detail section of the first electrode suspended over therod;

FIG. 3 is a partially sectional view of an embodiment of the apparatuswherein a second electric circuit is provided to create a magnetic fieldurging said plasma arc towards the center of the upper surface.

FIG. 4 is a detail sectional view of an embodiment with a refractoryguard ring arranged to prevent solid impurities such as casting powderfrom reaching the upper surface under the plasma arc;

FIG. 5 is a detail sectional view of an embodiment arranged to provide astream of inert gas for preventing oxidation of the molten metal;

FIG. 6 is a photographic view of a section of a slab cast byconventional methods; and

FIGS. 7 a and b are photographic views of a section of a slab cast fromthe same material using the method and the apparatus of the presentinvention.

FULL DESCRIPTION OF THE INVENTION

There is seen in FIGS. 1 and 2 a continuous casting apparatus 10 for theproduction of improved-quality rods 12 from molten metal 14.

A water-cooled mold 16 has a cavity open at its upper end 20 to receivemolten metal 14 and open at its lower end 22 to release the partiallysolidified rod 12 being formed. The mold 16 is electrically conductive,and functions as one of the components of a electric circuit.

A first plasma arc electrode 24 is positioned so that its lower face 26thereof is disposed in proximity to the upper surface 28 of the moltenmetal 14 having been poured into the mold 16.

A stand 30 is provided for supporting the first plasma arc electrode 24above the upper surface 28, leaving a gap of between 2-20 mm between thefirst electrode 24 and the upper surface 28 of the molten metal 14.

A second plasma arc electrode 32 is attached to the electricallyconducting mold 16 or to the metal 12 or 14.

Control means 34 are connected between the apparatus 10 and power supplysource 36. The control means 34 controls the electric circuitparameters, thus enabling an operator to increase or decrease power inaccordance with the needs of the process. The plasma arc 38 is formedbetween the upper surface 28 of the molten metal 14 and the lower face26 of the first electrode 24. The plasma arc 38 completes an electriccircuit, and causes stirring of the molten metal 14 in the mold 16.

The rod 12 continuous to grow in length as long as required and as longas further molten metal 14 is available from the tundish 46 through thenozzle 48.

With reference to the rest of the figures, similar reference numeralshave been used to identify similar parts.

Referring now to FIG. 3, there is seen a continuous casting apparatusfurther provided with a second electric circuit 68, requiring less powerthan the first electric circuit, is provided to create a magnetic fieldurging the plasma arc 38 (see FIG. 2) towards the center of the uppersurface 28 (see FIG. 2) of the molten metal 14.

The second electric circuit 68 passes through the molten metal held inthe tundish 46 and through the nozzle 48 and reaches the molten metal 14in the mold 16.

FIG. 4 shows a detail of a continuous casting apparatus 10, furtherprovided with a refractory guard ring 54 arranged to prevent solidimpurities such as casting powder 56 from the upper surface 28 under theplasma arc 38; The refractory guard ring 54 submerged to a level belowthe upper surface 28 of the molten metal 14 contained in the mold 16.

FIG. 5 shows a detail of a continuous casting apparatus 10, furtherprovided with a stream of inert gas 76 device for preventing oxidationof the molten metal 14; The first electrode 24 is hollow and a jet tube78 is provided for directing a stream of an inert gas 76, The gas jet 78impacts the upper surface 28 of the molten metal 14 of the rod 12 in themold 16. The inert gas prevents oxidation of the molten metal formingthe upper surface 28, and removes therefrom any solid impurities,particularly any casting powder if present.

Any combination of the embodiments described and illustrated in FIG. 3,FIG. 4 and FIG. 5 can be applied.

FIG. 6 is a photograph 90 of a copper-nickel slab section of 400*200 mmcast by conventional semi-continuous casting.

FIGS. 7 a and b are photographs of the same alloy been treated by themethod and apparatus of the present invention. The improvedmacrostructure is evident.

The scope of the described invention is intended to include allembodiments coming within the meaning of the following claims. Theforegoing examples illustrate useful forms of the invention, but are notto be considered as limiting its scope, as those skilled in the art willreadily be aware that additional variants and modifications of theinvention can be formulated without departing from the meaning of thefollowing claims.

1. A continuous and semi-continuous casting apparatus for the productionof improved-quality rods, including rod cross-sections having a aspectratio exceeding 1.8, the apparatus comprising: a) a stand for suspendingat least one electric arc electrode over the upper surface of the moltenmetal casting after or during pouring; b) at least one electrode forforming a moving electric arc over the upper surface of said metalliccasting being cast: c) a second electrode, being the liquid metal, forcompletion of an electric circuit including said electric arc and theliquid metal; and d) control(s) mean(s) connected between said apparatusand a power supply and arranged to supply power to the plasma arcelectrode(s) and monitoring the parameters of the electric arc.
 2. Thecontinuous and semi continuous casting apparatus as claimed in claim 1,wherein a second electric circuit is provided to create a magnetic fieldurging said plasma arc towards the center of said upper surface of saidmolten metal, by said second electric circuit passes through moltenmetal held in a reservior and reaches said metallic component of saidmold through a molten metal column formed in a feed tube.
 3. Thecontinuous and semi continuous casting apparatus as claimed in claim 1,wherein said first electrode is hollow and means are provided fordirecting a stream of an inert gas, there through to impact said uppersurface for preventing oxidation of the molten metal forming said uppersurface and for the removal therefrom of solid impurities and theremoval of casting powder if present.
 4. The continuous and semicontinuous casting apparatus as claimed in claim 1, further including arefractory guard ring slightly deeped in the surface of said rod tomaintain exclusion of solid impurities such as casting powder from saidupper surface under the plasma arc, and to electrically insulate anupper portion of said mold from said plasma arc.
 5. A continuous andsemi continuous casting apparatus substantially as describedhereinbefore and with reference to the accompanying drawings.
 6. Amethod for continuous and semi continuous metal casting for improvingquality of metallic continuous rods, said method comprising; step a)continuously pouring the liquid metal into a mold for casting rods. stepb) providing at least one first electric arc electrode and positioningsame slightly above the upper surface of the molten metal; step c)providing a second electrode in electrical contact with a section of themold being electrically conductive, and applying an electric current tosaid electrodes to form an arc between said first electrode and saidupper surface; and step d) continuously moving said electric arc(s) oversaid upper surface.
 7. A continuous or semi continuous casting method asclaimed in claim 6 including the use of casting powder by: step a)continuously pouring a liquid metal into a mold and applying castingpowder to the rod; step b) removing the casting powder from the arcworking zone. One of the ways to remove the casting powder is byblasting an inert gas such as argon over said upper surface; step c)preventing the return of said casting powder by placing at list onerefractory guard ring on said upper surface; step d) providing at leastone plasma arc electrode and positioning same slightly above said uppersurface of the molten metal, inside each protecting ring; step f)providing a second electrode in electrical contact with a section of themold being electrically conductive, and applying an electric current tosaid electrodes to form an arc between said first electrode and saidupper surface; and step e) continuously moving said electric arc(s) oversaid upper surface.
 8. A continuous or semi continuous casting method asclaimed in claim 6, wherein said electric current applied to saidelectrodes to form a plasma is a DC current.
 9. A continuous casting orsemi continuous method, substantially as described hereinbefore and withreference to the accompanying drawings.